As digital cameras have become increasingly popular nowadays, the importance of image processing to be performed on images shot with those digital cameras has been increasing year after year. There are various kinds of such image processing, examples of which include (i) compensation for backlight, which has been a major cause of a failure in making a good shot with a camera, (ii) increasing the resolution of a so-called “digitally zoomed” image, (iii) recognizing a human face or something like that and automatically focusing on it, and (iv) superposing a virtual image, generated by computer graphics, on a real image as known as “Augmented Reality”.
Each of these kinds of image processing is carried out based on the “appearance” of an object of shooting. Light that emitted from a light-source, is then reflected from the surface of the object, and finally received by an imager and the “appearance” of the object comes out. That is why the light-source information is very important in image processing, which means that it is very effective to obtain light-source information and utilize it for shooting and image processing. For example, there is a technique often used to give a natural 3D appearance to an object, by putting the object under semi-backlight on purpose. However, it heavily depends on the specific situation whether or not that special type of shooting can be done successfully. Consequently, in most cases, it is the shooter's instinct or experience that makes the difference.
Once the light-source information is known, it is possible to let the camera give instruction to the shooter, for example an instruction about the shooting direction, so as to allow him or her to copycat such a technique referred above to achieve a good shot. On top of that, it is also possible to make an automatic exposure correction based on the light-source information. Patent Document No. 1 discloses a technique for detecting a light-source with a sensor arranged at the top of a camera and instructing the shooter a recommended shooting direction. According to the technique disclosed in Patent Document No. 1, a photoelectric conversion element with a fish-eye lens is used as such a sensor for detecting the light-source direction. Specifically, the light-source direction is determined by the location of the point which has the maximum intensity on the sensor, where the light that comes from the whole sky gets condensed. In that case, however, if the sunlight reflected on a window is seen at a high position, there will be another intense light-source other than the sun itself and the detection of the sun direction could fail.
Patent Document No. 2 proposes a technique to use the whole sky polarization state in order to obtain the information about the position of the sun more accurately. According to the technique disclosed in Patent Document No. 2, a light-source detection section is arranged at the top of a camera as in Patent Document No. 1 to capture whole sky polarization images using the sensor that can capture the whole sky with a polarization filter attached to it. A number of images are shot with continually rotating the polarizer, and the whole sky polarization property is obtained from those images, thereby the sun direction is determined. Sky polarization is also discussed in Non-Patent Document No. 1, which proposes observing polarization state of sunlight using a fish-eye lens camera that can shoot wide areas of sky as in Patent Documents No. 1 and 2 cited above. Also, although no specific technique is disclosed, Non-Patent Document No. 1 also says that the sun direction could be determined based on the polarization state.
Patent Document No. 3 discloses a patterned polarizer for capturing multiple polarization images with mutually different polarization principal axes.
Also Non-Patent Documents No. 2, 3 and 4 refer to sky polarization patterns.